Tissue perfusion sensor and placement device

Abstract

A sensor system for measuring partial pressure of carbon dioxide is provided. The sensor system includes a biosensor and a sensor placement tool. The sensor includes a sensor cover having a generally C-shaped cross section defining an opening on its underside, a membrane body contained within the opening, the membrane body comprising an amorphous fluoroplastic, having a first end and a second end, and defining a chamber therein, a sensor body for coupling the membrane to the sensor cover, two or more electrodes disposed within the chamber of the membrane, and a substantially electrolyte-free liquid contained within the chamber of the membrane and in contact with the two or more electrodes. The sensor placement tool is configured to position the membrane body against the cheek tissue of the subject such that at least 40-50% of the membrane body is in contact with the cheek tissue.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Patent Application No. 17 / 230,020, filed April 14, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to the field of sensors for measuring the partial pressure of carbon dioxide (pCO2) in tissue. More particularly, the present disclosure relates to sensors for measuring the partial pressure of carbon dioxide in mucosal tissue. [Background technology]

[0003] Very low blood flow, known as hypoperfusion, can be caused by low blood volume, inadequate pumping of the heart, or excessive dilation (enlargement) of blood vessels.

[0004] The body responds to such stress by reducing blood flow to less vital organs, such as the gastrointestinal tract, in order to conserve blood for other more vital organs. Thus, when there is a reduction in blood flow from the heart, the body sends more blood to vital organs, such as the brain, which cannot survive long without a continuous supply of blood, while restricting blood flow to less vital organs, whose survival is not threatened by a temporary, significant reduction in blood flow.

[0005] For example, when there is a reduction in blood flow from the heart, blood flow to the splanchnic vasculature supplying the stomach and intestines, as well as to the esophagus and oral / nasal cavities, is greatly reduced. Therefore, reduced blood flow to the splanchnic vessels is an indicator of hypoperfusion in the patient. Hypoperfusion can compromise the intestinal mucosa, leading to ischemia and hypercapnia in the stomach. These two clinical conditions can result in the release of bacteria and inflammatory substances into the splanchnic circulation, potentially leading to sepsis and multiple organ dysfunction syndrome.

[0006] Metabolic carbon dioxide production continues in tissues even under conditions of low blood flow. Because carbon dioxide is not readily transported away, it accumulates and increases in concentration in tissues with low blood flow. This carbon dioxide accumulation is manifested by an increase in pCO2 within the organ. Therefore, hypoperfusion is commonly assessed by measuring pCO2 at these sites.

[0007] Increases in pCO2 can be measured throughout the body. In particular, studies have shown that oral mucosa pCO2 correlates well with gastric pCO2, making it an ideal site for measuring pCO2, especially if the sensing probe is isolated from ambient air and can be placed in the patient's mouth with minimal discomfort. Numerous studies have demonstrated that both sublingual and buccal mucosa pCO2 levels quantitatively trace circulatory stress.

[0008] Traditionally, pCO2 measurements have been performed with sensors that have silicone membranes. Silicone membranes are useful because they allow rapid gas transport due to the large free volume of the polymer chains. Unfortunately, however, silicone membranes also allow the passage of carboxylic acids, such as acetic acid, and other compounds found in saliva, which can interfere with pCO2 measurements. For example, when acetic acid passes through the membrane into the sensor fluid, it decreases the pH and increases the fluid's conductivity. Both changes can falsely indicate an increase in carbon dioxide.

[0009] Furthermore, measuring the partial pressure of gases within tissue requires that the sensor-tissue interface be isolated from the ambient air without applying excessive pressure. This has been attempted in several ways, but all have limitations. First, measurements at the external epidermis have used adhesive patches and gels to isolate and capture the gas environment. This method is not practical for use with oral mucosal tissue, which is inherently moist.

[0010] Second, handheld devices are used sublingually, where the tongue helps block exposure to the environment. This method is user-dependent, prone to error, and impractical for widespread application. Furthermore, Anderson (U.S. Patent No. 8,996,090) has proposed a method in which the device is constructed from a material that deforms in response to pressure. Anderson's method relies not only on the choice of material but also on the design of the applicator. Achieving the appropriate amount of flexibility—enough pressure to maintain contact without excessive pressure but sufficient to maintain sensor contact—is problematic for the range of cheek tissue thicknesses encountered in adult patients. Normal cheek tissue thickness can range from approximately 7 mm to 20 mm. Maintaining contact without impeding capillary blood flow requires a pressure of 25 mmHg or less. Pressures greater than 25 mmHg can cause blockage of blood flow, which can lead to measurement errors and tissue damage. Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, there is a need for new designs that allow rapid transmission of carbon dioxide while preventing transmission of low molecular weight acids found in saliva. There is also a need for tissue placement devices designed to hold and position sensors against tissue, such as mucosal tissue. [Means for solving the problem]

[0012] The above problems are addressed by the carbon dioxide sensor and tissue placement device of the present disclosure.

[0013] In one or more scenarios, a sensor system for measuring carbon dioxide partial pressure in tissue is disclosed. In one aspect, the sensor system may include a sensor and a sensor placement device. The sensor may include a sensor cover having a generally C-shaped cross section defining an opening on its underside, a membrane contained within the opening, including an amorphous fluororesin, having a first end and a second end and defining a chamber therein, a sensor body for coupling the membrane body to the sensor cover, two or more electrodes disposed within the membrane body, and a substantially electrolyte-free liquid contained within the chamber of the membrane body and surrounding the two or more electrodes. Optionally, the sensor may measure the impedance of the substantially electrolyte-free liquid in response to an alternating current potential. In various embodiments, a lip of the generally C-shaped sensor cover may be configured to shield the membrane body from end-tidal carbon dioxide. The sensor placement device may be configured to position the membrane body against the subject's cheek tissue so that at least 40-50% of the membrane body is in contact with the cheek tissue.

[0014] Optionally, the sensor placement device may include a sensor arm configured to couple to a sensor at a first end disposed on a first plane and a second arm including a deflection surface disposed on a second plane. The first plane is offset from the first plane by at least 5 mm. The sensor placement device may also include a beam for coupling the second end of the sensor arm to the second arm. The offset between the first plane and the second plane may be configured to accommodate the cheek tissue of the subject to position the membrane body against the cheek tissue. In certain embodiments, the membrane body may be either cylindrical or spherical. Alternatively and / or additionally, the deflection surface may include a U-shaped portion formed by two arms equidistant from the sensor coupled to the first end of the sensor arm. In such an embodiment, the sensor placement device may position the sensor against the cheek tissue without any gap and without applying a pressure exceeding 25 mmHg by folding the cheek tissue over the membrane body via the two arms of the U-shaped portion. Optionally, the beam of the sensor placement tool can include a ratchet element configured to vary the offset between the second plane and the first plane.

[0015] In some other scenarios, a sensor placement device for placing a sensor for measuring the partial pressure of carbon dioxide (pCO2) against cheek tissue of a subject is disclosed. The sensor placement device may include a sensor arm configured to couple to a sensor at a first end located on a first plane, a second arm located on a second plane including a U-shaped deflection surface, and a beam for coupling the second end of the first sensor arm to the second arm. The first plane may be offset from the second plane, and the offset may be configured to accommodate the cheek tissue of the subject to place the sensor membrane against the cheek tissue by folding the cheek tissue over the membrane via the U-shaped deflection surface. Optionally, the offset may be at least about 5 mm. Additionally and / or alternatively, the beam may include a ratchet element configured to vary the offset between the second plane and the first plane.

[0016] In various embodiments, the sensor placement tool can be configured to place the sensor against cheek tissue without any gaps and without applying pressure greater than 25 mmHg.Optionally, the U-shaped deflection surface can include two arms that are positioned equidistant from the sensor when coupled to the first end of the sensor arm.

[0017] In another scenario, a method for determining the partial pressure of carbon dioxide (pCO2) in tissue is disclosed. The method can include providing a sensor that can include a membrane housed within an opening formed by a C-shaped sensor cover, using a sensor placement tool to position the sensor near cheek tissue of a subject without any air gaps and without applying a pressure greater than 25 mmHg, and measuring pCO2 in the cheek tissue. The membrane can include a first amorphous fluororesin and form a sealed chamber with a first end and a second closed end.

[0018] In certain embodiments, the method may also include coupling a sensor to a first end of a sensor arm of the sensor placement instrument. The first end may be in a first plane offset from a second plane including a second arm of the sensor placement instrument that includes a U-shaped deflection portion. Optionally, the method may include controlling the offset between the first and second planes to be approximately 5 mm. Additionally and / or alternatively, the method may include controlling the offset between the first and second planes so that the two arms of the U-shaped deflection portion fold the cheek tissue onto the membrane without applying a pressure greater than 25 mmHg. Controlling the offset may include moving the second arm relative to the first end using a ratchet element included in the sensor placement instrument.

[0019] The method may optionally include shielding the membrane body from end-tidal carbon dioxide by providing a lip on the sensor cover having a generally C-shaped cross section and positioning the second closed end adjacent the lip.

[0020] These and other aspects of the present disclosure are set forth in the detailed description and appended claims.

[0021] For a better understanding of the present disclosure and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a side view of a carbon dioxide sensor according to the present disclosure. [Figure 2] FIG. 1 is a perspective view of a carbon dioxide sensor according to the present disclosure. [Figure 3] FIG. 2 is a bottom view of a carbon dioxide sensor according to the present disclosure. [Figure 4] FIG. 4 is a cross-sectional view of a carbon dioxide sensor according to the present disclosure taken along line AA of FIG. 3. [Figure 5] FIG. 1 is a side view of a sensor placement tool for placing a carbon dioxide sensor against an inner surface of the oral cavity. [Figure 6]FIG. 1 is a perspective view of a sensor placement tool for placing a carbon dioxide sensor against an inner surface of the oral cavity. [Figure 7] FIG. 1 is a perspective view of a sensor placement tool being used on a patient. [Figure 8] 1 is a graph showing a comparison of exposure of various membrane materials to acetic acid. [Figure 9] 1 is a graph showing a comparison of a silicone film and an amorphous fluororesin film in tissue. [Figure 10] FIG. 1 is a side view of a sensor placement tool according to the present disclosure. [Figure 11] FIG. 10 is a bottom view of a sensor placement tool according to the present disclosure. [Figure 12] FIG. 1 is a perspective view of a sensor placement tool according to the present disclosure. [Figure 13] FIG. 10 is a side view of an alternative ratcheting version of a sensor placement tool according to the present disclosure set at 5 mm spacing. [Figure 14] FIG. 10 is a partial view of an alternative ratcheting version of a sensor placement tool according to the present disclosure set at 5 mm separation. [Figure 15] FIG. 10 is a side view of an alternative ratcheting version of a sensor placement tool according to the present disclosure set at 10 mm separation. [Figure 16] FIG. 10 is a partial view of an alternative ratcheting version of a sensor placement tool according to the present disclosure set at 10 mm separation. DETAILED DESCRIPTION OF THE INVENTION

[0023] In the drawings, like reference numbers indicate identical or corresponding parts throughout the several views.

[0024] As used herein, the words "a," "an," and the like generally have the meaning of "one or more," unless otherwise specified. The term "plurality," as used herein, is defined as two or more than two. The term "another," as used herein, is defined as at least a second or more. The terms "comprises" and / or "having," as used herein, are defined as including (i.e., open language).

[0025] Throughout this document, references to "one embodiment," "particular embodiment," "embodiment," "implementation," "example," or similar terms mean that a particular feature, structure, or characteristic described in connection with an example is included in at least one example of the disclosure. Thus, the appearances of such language in various places throughout this specification are not necessarily all referring to the same example. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more examples, without limitation.

[0026] The term "or" as used herein should be interpreted as meaning an inclusive or, or any one or any combination. Thus, "A, B, or C" means "any of A; B; C; A and B; A and C; B and C; A, B, and C." Exceptions to this definition occur only when combinations of elements, features, steps, or operations are inherently mutually exclusive in some manner.

[0027] Additionally, in the individual figures, some of the components / features shown are drawn to scale to illustrate particular implementations, while other components and features are not drawn to scale.

[0028] 1, a side view of a carbon dioxide sensor according to the present disclosure is shown. The sensor 10 generally includes a sensor cover 12, a sensor body 14, an electrode pair 16, a membrane body 18, a sensor liquid 20, and a wound filament 22.

[0029] The sensor cover 12 may be constructed of a thermoplastic, such as polyethylene, polypropylene, polystyrene, or polycarbonate. The sensor cover 12 generally covers and accommodates the sensor body 14 and membrane 18. The sensor cover 12 is shaped to form an opening 26 on its underside to accommodate the sensor body 14 and membrane 18. One such shape is generally C-shaped in cross section. The sensor cover 12 includes a downwardly projecting lip 24 that shields the membrane 18 from end-tidal carbon dioxide during use. Advantageously, the sensor cover 12 allows 40% to 50% or more of the membrane 18 to contact tissue. The sensor cover 12 is designed to cover the portion of the membrane 18 that does not contact tissue. A cylindrical membrane 18 is used because the sensor placement tool 58 is designed to fold the tissue around one side of the sensor. To cover the portion of membrane 18 that does not contact tissue, the inner surface of sensor cover 12 may have a C-shaped cross section to minimize gaps (dead space that acts as a sink that can slow the sensor response). Additionally, the sides of sensor cover 12 may be tapered to provide better contact with tissue.

[0030] The sensor body 14 may also be constructed of thermoplastics such as polyethylene, polypropylene, polystyrene, and polycarbonate. The sensor body 14 may be constructed of the same thermoplastic as the sensor cover 12, or may comprise a different thermoplastic. Preferably, for manufacturing cost efficiencies, the sensor body 14 is constructed of the same thermoplastic as the sensor cover 12. The sensor body 14 is configured to securely hold and align the electrodes 16 in place within the membrane body 18. The sensor body 14 provides attachment points for the membrane body 18 and for securing the wound filament 22 to provide a secure attachment between the sensor body and the membrane body 18. Those skilled in the art will appreciate that other attachments, such as snap-on, adhesive, glue, and crimping, may also be used.

[0031] The electrode pair 16 is constructed of stainless steel and is configured to receive an AC potential from a power source. Those skilled in the art will understand that metals other than stainless steel may be used. The electrodes 16 are securely held in place by the sensor body 14. The electrodes 16 are housed within a membrane body 18 and disposed within a sensor solution 20. Those skilled in the art will understand that more than two electrodes may be used and still be within the scope of the present disclosure. For example, conductance may be measured with two, three, or four electrodes.

[0032] The membrane 18 is positioned within the opening 26 of the sensor cover 12. The membrane 18 comprises a hollow tube defining an internal chamber. The membrane 18 is substantially impermeable to low-molecular-weight carboxylic acids, including acetic acid, found in salvia, which can impair accurate readings of carbon dioxide levels in the oral mucosa. The membrane 18 may be composed of a fluoropolymer resin, such as an amorphous fluoropolymer. Suitable amorphous fluoropolymers include Teflon AF2400 (available from The Chemours Company). Teflon AF2400 is known to have excellent permeability to carbon dioxide. However, it has not previously been discovered that amorphous fluoropolymers such as Teflon AF2400 have a structure with large free volume in the polymer chains that allows rapid transport of carbon dioxide but not carboxylic acids such as acetic acid. Compared to polytetrafluoroethylene, which has a carbon dioxide permeability of 120 Barrer units, Teflon AF2400 has a carbon dioxide permeability of 2800 Barrer units. Alternatively, polymethylpentene (available from Mitsui Chemicals America) may be used instead of the amorphous fluororesin. The membrane body 18 is open at the first end 28 so that it can be filled with the sensor liquid 20 before attachment to the sensor body 14, after which it is sealed. The second end 30 is sealed with Teflon AF1600, which has a much lower carbon dioxide permeability than AF2400. Teflon AF1600 easily fuses to the membrane body 18, providing a leak-tight environment. Because the second end of the tube is positioned against the lip 24, it does not come into contact with tissue and does not need to be carbon dioxide permeable. The sensor liquid 20 can be a liquid that is substantially electrolyte-free, such as pharmaceutical-grade purified water (USP-grade water). In some embodiments of the present disclosure, distilled water can also be used.

[0033] The wound filament 22 is used to secure the membrane body 18 to the sensor body 12. An adhesive may be used to bond and reinforce the wound filament 22.

[0034] 5-7, a sensor placement device 50 for securely positioning the sensor 10 against the interior surface of the oral cavity is illustrated. The sensor placement device generally includes a proximal end 52, an elongated intermediate section 54, and a distal end 56. The proximal end 52 is adapted to operably couple to an electronic device for reading and displaying pCO2 measurements. The distal end 56 includes a U-shaped ridge 56 for positioning the device 50 against the outer surface of the cheek and positioning the sensor against the inner surface of the cheek. The sensor 10 is attached to the placement device 50 by an arm 58.

[0035] In operation for use in measuring pCO2 in tissue within the oral cavity, the sensor 10 is mounted on a sensor placement device 50 configured to fit against a human cheek. As shown, the sensor placement device 50 with the sensor 10 is a disposable device. Using the elongated midsection, a user can insert the sensor 10 into a subject's mouth and position the U-shaped ridge at the distal end 56 against the outer surface of the cheek, with the subject's cheek positioned between the arm 58 containing the sensor 10 and the U-shaped ridge at the distal end 56. This holds the sensor 10 between the two arms of the U-shaped ridge against the inner oral surface of the cheek, such that the two arms of the U-shaped ridge fold the cheek tissue (i.e., the inside of the cheek) against the membrane (e.g., cylindrical, dome-shaped, etc.) of the sensor 10 to provide optimal contact without applying excessive pressure. The device 50 is designed (i.e., the spacing between the sensor face and the U-shaped ridge is set) to keep the sensor 10 in direct contact with the cheek tissue without any air gaps and without applying pressure exceeding 25 mmHg, preferably less than 20 mmHg, less than 15 mmHg, etc. Excessive pressure can impede blood flow and alter pCO2 levels. One skilled in the art will appreciate that the U-shaped ridge can be any suitable shape, such as a V-shape, a C-shape, a square loop, a triangular loop, an oval loop, etc.

[0036] The response time of the sensor 10 for measuring pCO2 can be affected by the ratio of the sensor's surface area (allowing analyte passage) to its volume. When the sensor is placed near a mucosal surface (no pressure is applied), only a small percentage of the sensor's cylindrical membrane is in direct (tangential) contact with the tissue, leading to increased response time. When the appropriate pressure is applied to press the cylindrical surface against the tissue, the membrane surface deflects away from the pressure, allowing approximately 40-50% of the membrane surface to contact the tissue. However, the applied pressure must be carefully adjusted to avoid impeding capillary blood flow and introducing errors into the measurement. The sensor placement device 50 of the present disclosure is configured to fold the cheek tissue around the cylindrical membrane surface of the sensor 10 to achieve greater contact with this tissue without applying excessive pressure. Optionally, the sensor cover may also taper away from the membrane surface to allow a higher percentage of tissue contact.

[0037] A sensor cable (not shown) attaches the sensor placement device with the sensor to electronics (not shown), which applies an AC potential to the sensor 10 and measures the impedance of the sensor solution 20 contained within the membrane 18. The device is calibrated to the sensor's response curve, and an algorithm calculates the pCO2 value from the temperature-adjusted conductance signal. The sensor's response curve is determined by measuring the sensor signal with two reference solutions of known pCO2 levels: a low pCO2 reference solution and a "standard" pCO2 reference solution. The "standard" solution approximates the pCO2 of healthy, well-perfused tissue. From this data, the slope of the response curve is determined. The pCO2 value is then calculated from the difference in signal from the "standard" reference solution. The calculated pCO2 value is then displayed graphically and numerically on an integrated display. While the electronic device is configured as a stand-alone patient monitoring device, those skilled in the art will understand that it can be integrated into a multimodal patient monitoring system.

[0038] Comparative data will now be discussed with reference to Figures 8 and 9. Figure 8 graphically illustrates the results of an in vitro study of several membrane materials. These membranes were exposed to an 8 mM acetic acid solution, and the change in conductance was observed to determine their suitability for intraoral application. The membrane thickness was selected based on its ability to achieve reasonable carbon dioxide permeability. The results demonstrate the superiority of Teflon AF2400. Figure 8 shows a comparison of potential membrane materials evaluated by the thickness required to provide comparable response times: A = 0.005 inch PDMS silicone; B = 0.001 inch Teflon AF2400; C = 0.0004 inch PTFE; D = 0.0005 inch FEP; and E = 0.015 inch FVMQ silicone.

[0039] Conductance probes were coated with the test material and then exposed to 8 mM acetic acid (within the physiological range of saliva). Among the materials tested, a typical membrane material (PDMS silicone) was the most permeable to acetic acid. At 60 minutes, conductance increased by 1 μS / cm due to acetic acid passing through the PDMS silicone membrane. Membrane E (0.015 inch FVMQ silicone) experienced an increase of 0.12 μS / cm over that time, while membranes C (0.0004 inch PTFE) and D (0.0005 inch FEP) showed better resistance to acetic acid permeation, approximately 0.05 μS / cm at 60 minutes. However, membrane B (0.001 inch Teflon AF2400) showed no detectable increase in conductance over the same time period.

[0040] Figure 9 graphically illustrates an overlay of an in vivo study of a sensor constructed with a silicone membrane compared to a sensor constructed with a Teflon AF2400 membrane. A reference solution was measured before and after exposure to oral mucosal tissue. The results demonstrate the potential contamination caused by the use of a silicone membrane and demonstrate the suitability of Teflon AF2400. Figure 9 shows a comparison of tissue data collected with a PDMS silicone membrane (A) and a Teflon AF2400 membrane (B). Reference values ​​were measured in a tonometer using 10% CO2 (pCO2 = approximately 70 mmHg) water. The sensor was then placed on the subject's cheek tissue, and data was collected for approximately 60 minutes. The graph shows that the Teflon AF2400 membrane sensor stabilized at approximately 53 mmHg pCO2, while the PDMS silicone membrane sensor continued to rise above 60 mmHg pCO2 and never stabilized. The sensor was then returned to the tonometer. The Teflon AF2400 membrane sensor returned to the value before tissue exposure, while the silicone membrane sensor showed an error of approximately 10 mmHg pCO2, which correlated with the increase in signal due to acetic acid contamination.

[0041] Referring now to FIG. 10 , a side view of a sensor placement device 50 according to the present disclosure is shown. The device 50 positions the sensor 10 against tissue. The sensor placement device 50 generally includes a curved sensor arm 58, an angled beam 113, a connecting post 114, and a deflection surface 115. The sensor placement device 50 provides sufficient space so as not to compress the tissue on which the sensor 10 is placed. Rather, the sensor placement device 50 (e.g., two arms of a U-shaped portion) utilizes the flexibility of cheek tissue to fold the tissue around the cylindrical membrane of the sensor 10 to achieve 40-50% or greater contact with the cheek tissue, as disclosed below. The device 50 includes a sensor arm 58 on one plane and two arms equidistant from the sensor arm 58 that together form a U-shaped deflection surface 115 on a separate plane, with the sensor 10 positioned between the two arms of the U-shaped deflection surface 115, as best seen in FIG. 11 . In this way, deformability of the material of the device 50 is not required.

[0042] Sensor 10 measures an analyte or a characteristic indicative of microcirculatory blood flow. The membrane of sensor 10 is preferably cylindrical or dome-shaped, or has an appropriate geometric feature adapted to fold over tissue. When measuring gas with sensor 10, sensor 10 requires sensor cover 12 to protect the sensor from exposure to ambient and end-tidal gases. Sensor arm 58 may be constructed of a thermoplastic, such as engineering thermoplastic polyurethane, polyethylene, polypropylene, polystyrene, and polycarbonate. Sensor arm 58 is attached to beam 113, which is attached to the top of post 114. In this manner, sensor arm 58 is configured to hold sensor 10 above sensor plane 117. Beam 113 and post 114 may also be constructed of a thermoplastic, such as engineering thermoplastic polyurethane, polyethylene, polypropylene, polystyrene, and polycarbonate.

[0043] A deflection surface 115, similarly constructed of a thermoplastic such as engineering thermoplastic polyurethane, polyethylene, polypropylene, polystyrene, and polycarbonate, is attached to the bottom of post 114. The bottom of deflection surface 115 thus defines deflection plane 118. To ensure contact with normal cheek tissue thickness (approximately 7 mm) at the bottom, sensor arm plane 117 and deflection plane 118 may be spaced apart by less than about 5 mm, about 4-6 mm, about 5 mm, about 3-5 mm, etc., so that the applied pressure does not exceed about 25 mmHg, which could cause obstruction of blood flow and error in the measured pCO2. Ideally, the line of sight above sensor 10 should be clear for at least 20 mm, at least about 10 mm, at least about 15 mm, about 15-25 mm, etc., to prevent tissue from becoming trapped between the surfaces of instrument 50. Optionally, to accommodate thicker cheek tissue, the inner circumference of deflecting surface 115 can be offset from the outer circumference of sensor 10 (shown in FIG. 11) by about 15 mm to about 20 mm, about 16 mm to about 19 mm, about 17 mm to about 18 mm, about 15 mm to about 20 mm, etc.

[0044] FIG. 13 illustrates a sensor placement tool 119 similar to that disclosed in FIG. 10 , but including a ratchet element 120 configured to vary and control the separation between the sensor plane 117 and the deflection plane 118. A ratchet element 120 corresponding to a separation between the sensor plane 117 and the deflection plane 118 of approximately 5 mm is shown in the partial view of FIG. 14 . The ratchet element 120 may include saw teeth 121 found on a post 122 and complementary ratchet teeth 123 on a beam head 124. The beam head 124 is further attached to a beam 125 and has alignment features that mate with the post 122. The beam head can move up and down the post 122 via engagement of the saw teeth 121 with the complementary ratchet teeth 123. FIGS. 15 and 16 illustrate increasing the separation between the sensor plane 117 and the deflection plane 118 via the ratchet feature 120. In this manner, the device 119 may be adaptable to a wide range of cheek tissue thicknesses. Other mechanisms for adjusting the spacing between the sensor plane 117 and the deflection plane 118 are within the scope of this disclosure.

[0045] Although the present invention has been described with reference to particular aspects and embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. Partial pressure of carbon dioxide in tissues (pCO ２ 1. A sensor system for measuring a temperature, comprising: a sensor and a sensor placement tool; The sensor a sensor cover having a generally C-shaped cross section defining an opening bounded at its top by an underside of the sensor cover and at its distal end by a lip; a membrane contained within the opening, the membrane including a first amorphous fluororesin, the membrane having a first end and a second closed end positioned against the lip to form a sealed chamber; a sensor body disposed at the first end for coupling the first end to the sensor cover; two or more electrodes extending from the first end and disposed within the sealed chamber; a substantially electrolyte-free liquid contained within the sealed chamber and in contact with the two or more electrodes; Including, the sensor placement device is configured to position the membrane against the cheek tissue of the subject so that at least 40-50% of the membrane contacts the cheek tissue; Sensor system.

2. The sensor placement tool includes: a sensor arm configured to couple to a sensor at a first end that is in a first plane; a second arm with a deflection surface in a second plane offset from the first plane by 3-6 mm; a beam for coupling the second end of the sensor arm to the second arm; an offset between the first plane and the second plane configured to accommodate cheek tissue of a subject to position the membrane body against the cheek tissue. The sensor system of claim 1 .

3. The sensor system of claim 2 , wherein the membrane is cylindrical or spherical.

4. The sensor system of claim 2 , wherein the deflection surface includes a U-shaped portion formed by two arms equidistant from a sensor coupled to a first end of the sensor arm.

5. 5. The sensor system of claim 4, wherein the sensor placement tool is configured to place the sensor against cheek tissue without gaps and without applying pressure greater than 25 mmHg by folding cheek tissue over the membrane via two arms of the U-shaped portion.

6. The sensor system of claim 2 , wherein the beam includes a ratchet element configured to vary the offset between the second plane and the first plane.

7. The sensor system of claim 1 , wherein the sensor measures the impedance of a substantially electrolyte-free liquid in response to an alternating current potential.

8. The sensor system of claim 1 , wherein the lip of the generally C-shaped cross section of the sensor cover is configured to shield the membrane from end-tidal carbon dioxide.

9. Partial pressure of carbon dioxide (pCO ２ a sensor placement device for placing a sensor against cheek tissue of a subject to measure a first arm having a first end configured to couple with a sensor through a sealed chamber formed by a membrane housed in a sensor cover and a second end configured to couple with a beam of the sensor placement device, the first end and the second end being spaced apart from one another; a second arm having a U-shaped deflection surface that lies in a second plane that is parallel to and offset from a first plane in which the sensor lies when coupled to the first end of the first arm, the second plane being located between the first plane and another parallel third plane in which a second end of the first arm lies; the beam for connecting the second end of the first arm to the second arm; wherein the offset between the first plane and the second plane is configured to accommodate cheek tissue of the subject to fold the cheek tissue onto the membrane via the U-shaped deflection surface, thereby positioning the sensor membrane against the cheek tissue. Sensor placement fixture.

10. The sensor placement tool of claim 9, wherein the offset is between 3 and 6 mm.

11. The sensor placement tool of claim 9 , wherein the beam includes a ratchet element configured to vary the offset between the second plane and the first plane.

12. 10. The sensor placement device of claim 9, wherein the sensor placement device is configured to place the sensor against cheek tissue without any air gaps and without applying pressure greater than 25 mmHg.

13. 10. The sensor placement tool of claim 9, wherein the U-shaped deflection surface includes two arms that are positioned equidistant from the sensor when coupled to a first end of the first arm.

14. Partial pressure of carbon dioxide in tissues (pCO ２ ) a method for determining providing a sensor including a membrane contained within an opening formed by a C-shaped sensor cover, the membrane including a first amorphous fluororesin and forming a sealed chamber having a first end and a second closed end; using a sensor placement tool to place the sensor adjacent to the subject's cheek tissue without any gaps and without applying pressure greater than 25 mmHg; The sensor was used to measure pCO ２ and measuring A method comprising:

15. 15. The method of claim 14, further comprising coupling the sensor to a first end of a sensor arm of a sensor placement fixture, the first end lying in a first plane offset from a second plane that includes a second arm of the sensor placement fixture that includes a U-shaped deflection portion.

16. 16. The method of claim 15, further comprising controlling the offset between the first plane and the second plane to be between 3 and 6 mm.

17. 16. The method of claim 15, further comprising controlling an offset between the first and second planes such that two arms of the U-shaped deflection portion fold cheek tissue onto the membrane without applying a pressure greater than 25 mmHg.

18. 18. The method of claim 17, wherein controlling the offset comprises using a ratchet element included in the sensor placement tool to move the second arm relative to the first end.

19. 15. The method of claim 14, further comprising shielding the membrane body from end-tidal carbon dioxide by providing a lip on the sensor cover having a generally C-shaped cross section and positioning the second closed end adjacent the lip.

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